The present invention is a method for the nonoxidative dehydrogenation of an alkylaromatic feedstream using a radial reactor. The radial reactor comprises a dehydrogenation catalyst bed having a dehydrogenation catalyst material layer and a layer of an inert material containing a potassium compound. The catalyst bed is divided into inner and outer ring-shaped layers, wherein catalyst material is placed in the outer layer and inert material containing a potassium compound is placed in the inner layer.
Radial reactors are known in the art and are utilized for a number of different types of catalytic reactions. For example, radial reactors are commonly utilized for the dehydrogenation of hydrocarbons, such as the dehydrogenation of acyclic and aromatic hydrocarbons to their correspondingly less saturated hydrocarbon products. One of the best known of these dehydrogenation processes is the conversion of ethylbenzene to styrene.
The conventional ethylbenzene dehydrogenation process (referred to as “nonoxidative dehydrogenation”) generally uses an iron oxide catalyst and the reaction takes place in the absence of oxygen. The conversion of ethylbenzene to styrene is an endothermic reaction which requires the addition of heat to the process to maintain an appropriate level of activity. In addition, it has been found that regulating the flow rate of the ethylbenzene as it passes across the catalyst bed, regardless of the thickness of the catalyst bed in the reactor assembly, can generally ensure that an acceptable level of selectivity and activity for the reactions is maintained. But even with such regulation the catalyst material will lose selectivity and activity over time.
The commercial process for the conversion of ethylbenzene to styrene is normally conducted in a series of radial, adiabatic reactors. It has been observed that a reactor system containing multiple radial reactors may produce a higher degree of conversion of the hydrocarbon and may have greater product yield than is exhibited by use of a single radial reactor. Thus, it is not uncommon for three or more radial reactors to be arranged in a serial flow orientation with reheat means, which may be located both within and between the reactors, to add heat to the reaction.
Conventional radial reactors contain an inlet located in the center of the radial reactor assembly. Catalysts for the reaction are placed within a bed or beds in the reactor assembly, generally occupying a ring-shaped, vertical space, which is located outside of a central core of the reactor. The feedstream enters the reactor through the inlet and then flows radially outward through the catalyst bed to an open, annular space, which is formed outside of the catalyst bed but within the reactor assembly. Ultimately the feedstream flows to an outlet as shown, for example, in U.S. Pat. No. 3,898,049.
In radial reactors used for ethylbenzene dehydrogenation, the gas feed flows radially from the central core of the reactor assembly through catalyst material contained in a ring-shaped, vertical catalyst bed contained within the radial reactor. However, because the nonoxidative dehydrogenation of ethylbenzene is a temperature sensitive reaction, the volume of the catalyst material within the catalyst bed that actually catalyzes the feedstream is often limited. For example, in nonoxidative dehydrogenation radial reactors for the dehydrogenation of ethylbenzene, only the first 4 inches (10 cm) to 15 inches (40 cm) or so of thickness of the catalyst material contained in the ring-shaped, vertical layer of the catalyst bed effectively dehydrogenates the ethylbenzene feedstream. This section of the catalyst bed also loses the greatest amount of the potassium during the dehydrogenation reaction. Because the reaction is adiabatic, by the time the ethylbenzene feedstream has passed about 18 inches (46 cm) or so through the catalyst bed, the temperature of the feedstream has dropped to such an extent that the activity of the reaction is diminished dramatically or even extinguished. Further, when the temperature of the ethylbenzene feedstream drops as it passes through a thick catalyst bed, a higher percentage of undesired by-products are produced. In addition, the greater the thickness of the catalyst bed, the greater the pressure drop as the feedstream passes through the catalyst bed.
Notwithstanding these reductions in the performance of the catalyst material in the thick catalyst beds contained in large diameter radial reactors, it has become conventional to build shorter reactor assemblies with thicker catalyst beds instead of building taller radial reactors with thinner catalyst beds because of the high cost in building both the support structure for the reactors and the radial reactors themselves. While shorter, thicker radial reactors contain the same overall quantity of catalyst material as taller, thinner radial reactors, the performance of these shorter, thicker reactors is not as efficient as when a taller, but narrower reactor is utilized.
Conventional nonoxidative dehydrogenation processes generally utilize a single dehydrogenation catalyst, such as a conventional iron oxide catalyst containing a small amount of potassium and chrome as disclosed, for example, in U.S. Pat. Nos. 2,866,790 and 2,866,791. Various catalysts for nonoxidative dehydrogenation are also disclosed in U.S. Pat. No. 6,191,065. The teachings of the '790 patent, the '791 patent and the '065 patent are incorporated herein by reference. These conventional catalysts when used for the conversion of ethylbenzene to styrene gradually deactivate during normal use, causing a reduction in ethylbenzene conversion. As part of the deactivation process, the catalyst loses potassium. During the dehydrogenation reaction, potassium migrates across the catalyst bed from the inlet side to the outlet side. Thus, the catalyst located closest to the inlet generally exhibits the greatest potassium loss over time.
Several attempts have been made to address the problem of loss of potassium from the dehydrogenation catalyst during a nonoxidative dehydrogenation reaction. For example, an alkali metal or alkali metal compound can be introduced continuously or intermittently to the reactant stream. Alternatively, the alkali metal compound can be added in the form of a dry solid powder, or a solid lump containing the alkali metal compound can be placed in the path of the heated reactant feedstream causing the lump to gradually vaporize during processing. However, these are generally unsatisfactory fixes for the overall problem of the potassium loss. Thus, there is a need for a more effective means of introducing or maintaining potassium in the catalyst bed for nonoxidative dehydrogenation reactions.